In the fan industry, efforts have been made to lower the acoustic level as air is impelled through an axial or centrifugal fan (referred to hereinafter as a "fluid impelling device"). In a fluid impelling device, due to high favorable pressure gradients that drive fluid flow toward an inlet passage, the flow along the surface of the impelling device tends to be laminar. It is well known in the industry that laminar flow has relatively low mean kinetic energy when it is in contact with a solid surface. Specifically, laminar flow behaves in a viscous manner so as to substantially slow fluid flow along the surface of the impelling device as it moves toward the inlet passage.
One type of fluid impelling device includes an abrupt 90 degree turn at a juncture between the surface of the impelling device and the inlet passage. As the fluid flow moves into the inlet passage, a flow separation normally occurs as a portion of the laminar fluid flow cannot redirect its path to conform along the inlet passage. Accordingly, the acoustic level around the inlet passage tends to increase in the presence of a flow separation.
At present, there are several available air inlet devices. One such device is disclosed in U.S. Pat. No. 3,814,538 (Sjoqvist). The device includes two different size cylindrical rings concentrically positioned relative to one another. The smaller inner ring, having a rectangular cross-section, is positioned within the inlet passage of an impelling device. The outer ring, also rectangular in cross-section, is disposed on the surface of the impelling device adjacent the inlet passage. In their respective positions, the inner and outer rings are substantially at right angles to the surface of the impelling device such that a toroidal whirling flow may be generated against the outer ring when fluid flows into the inlet passage. The whirling flow, however, only acts to decrease the friction loss coefficient so as to make the air flow is less viscous across the impelling device.